<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            margin: 0;
            height: 100vh;
        }
        .left-sider {
            width: 100px;
            height: 100vh;
            background: red;
            float: left;
        }
        .right-sider {
            width: 100px;
            height: 100vh;
            background: green;
            float: left;
        }
        .container {
            /* 核心 */
            width: calc(100% - 200px);

            height: 100vh;
            background: blue;
            float: left;
        }
    </style>
</head>
<body>
    <div class="left-sider"></div>
    <div class="container"></div>
    <div class="right-sider"></div>
</body>
</html>